The invention relates to a polyaxial bone anchoring device, in particular for use in spinal or trauma surgery. The polyaxial bone anchoring device comprises a bone anchoring element with a shank to be anchored to a bone and a head. The head is pivotably held in a receiving part and can be locked at an angle relative to the receiving part by applying pressure onto it via a pressure member. With the receiving part, the bone anchoring element can be coupled to a stabilization rod that can be secured by a locking member. The pressure member comprises a deformable portion. First, the locking member applies a load to the pressure member that deforms the deformable portion whereby the head is locked. Thereafter the locking member comes into contact with the rod and locks the rod. The polyaxial bone anchoring device permits locking the head and the rod in a sequential manner, using a tool with a single drive portion that engages the locking member.
U.S. Pat. No. 8,088,152 B1 describes an orthopedic retaining system comprising at least one bone screw which has a head part and a shaft pivotably mounted thereon. A clamping sleeve is mounted in the head part, which can be pressed against the threaded shaft from its upper side to secure the threaded shaft relative to the head part. Further, a clamping device is provided by means of which the clamping sleeve and a retaining bar are pressed into the head part. The clamping device comprises an elastically deformable pressure member which is displaced into a clamping position during actuation of the clamping device.
US 2012/0253408 A1 describes a bushing for a bone screw assembly that includes a distal portion, a receiver portion, and a proximal portion. The distal portion includes a distal engagement surface that engages with a head of a bone screw. The receiver portion receives a fixation rod. The proximal portion includes a proximal engagement surface that engages with a locking member and a deformable portion that deforms from a first configuration to a second configuration based on a force applied by the locking member.
US 2013/0345761 A1 describes a polyaxial bone anchoring device comprising a bone anchoring element with a shank and a head and a receiving part pivotably coupled to the shank and having a channel for receiving a rod. A pressure member is arranged in a receiving part and is configured to exert pressure onto the head to lock the head in a receiving part. The pressure member has a deformable portion. Load applied to the pressure member by a locking member clamps the head and thereafter contacts the pressure member such that the deformable portion is deformed and the locking member comes into contact with the rod and clamps the rod.
While known devices allow locking of the head of a bone anchoring device in the receiving part first and thereafter locking of the rod with a single instrument in a sequential manner, the necessary locking force for locking the devices is high. This results from the fact that the final step of clamping and locking the rod involves both the deformation of a deformable portion of the pressure member as well as a force applied for clamping and locking the rod. The force necessary for the deformation is lost for the clamping. Therefore, a relatively high force has to be applied to safely lock the rod.